The present invention relates to a battery pack for a portable electric apparatus, comprising at least one battery cell and terminals for electric connection to the apparatus, and furthermore the present invention relates to a method for charging such a battery pack.
Battery packs of the kind described above are used as power supplies for e.g. wireless radio telephones, such as mobile telephones. Normally, such a battery comprises a number of battery cells with a certain terminal voltage, said cells being connected in series with each other so as to provide a supply voltage suitable for driving the electric and electronic components in the mobile telephone. Common battery cell types are NiCd and NiMH. If for instance five NiMH cells are used, a supply voltage of between 5 and 6 V may be obtained. The development of more compact and less power-consuming components has opened up the possibility of driving the electronic circuits in the mobile telephone at a lower supply voltage than before. Consequently, NiMH batteries with for instance four cells may be used, thereby providing a supply voltage of about 4 V. Three NiMH cells will provide a supply voltage of about 3 V, which is sufficient to drive modern CMOS integrated circuits.
Another type of batteries uses lithium-based cells. A lithium cell has a terminal voltage of around 3.6 V but should be charged at a higher voltage of around 4.2 V. As a consequence, a lithium battery with two cells must be charged at a relatively high voltage of about 8.5 V, while the electronic circuits in the mobile telephone only require a supply voltage, which is approximately half as large as aforesaid voltage.
Throughout this document the invention will be exemplified as a battery pack for a mobile telephone in any common cellular mobile telecommunication system, such as GSM. However, it is to be emphasized that the invention is applicable also for other types of portable battery-powered electric apparatuses, the batteries of which are arranged to be charged during ongoing apparatus operation. Examples of such electric apparatuses are portable computers (so called lap-top or palm-top computers), personal digital assistants (PDA), hand-carried communication units, etc.
FIG. 1 illustrates a conventional arrangement for charging a battery pack 10 with battery cells 11a, 11b for a mobile telephone 20 during ongoing operation. The mobile telephone 20 comprises electronic circuits 22, which require a certain supply voltage of, say, z V. The electronic circuits 22 comprise components such as power amplifiers, local oscillator, mixer, filter circuits, etc, said components being well-known in the technical field and are therefore not described in more detail herein. The battery pack 10 has a terminal voltage of y V, which is dependent on the cell voltage of the individual battery cells 11a, 11b. The battery has to be charged at a voltage x V, where x as described above is higher than y, which in turn may be considerably higher than z. Furthermore, the battery pack 10 comprises a safety device 12, the purpose of which is to prevent the battery cells 11a, 11b from being exposed to an excessively high voltage from a charging device 30. Other safety devices may be arranged for corresponding protection of the electronic circuits 22.
The arrangement according to FIG. 1 has a disadvantage in that the voltage-sensitive electronic circuits 22 may accidentally be exposed to dangerous voltage levels, if for some reason a fault would occur anywhere in the arrangement. For instance, the battery may unintentionally be removed from the mobile telephone. The safety device may comprise a fault, which prevents the safety device from releasing in response to an occurred fault condition. In such a case there is an apparent risk that the electronic circuits 22 will be damaged in response to an excessively high supply voltage of x V from the charging device 30.
Another disadvantage of a conventional charging arrangement according to the above is that a given mobile telephone model is generally only able to use batteries of a certain type. One reason for this is that different battery types have completely different charging characteristics during the charging process. For instance, NiMH-type batteries have voltage characteristics, which are schematically illustrated in FIG. 2. During constant current charging the terminal voltage u of the battery increases with time t, as shown in FIG. 2. When the battery approaches a fully charged state, at a time t.sub.f, the voltage characteristics exhibit an increased terminal voltage u during a short period of time, followed by a decreased terminal voltage u. This fact, i.e. the fact that the battery terminal voltage exhibits a negative voltage derivative, when the battery has reached a fully charged state, is commonly used for controlling the charging process. According to this method, which is referred to as "-dV" (minus delta volt), the charging is aborted, once the characteristic change in sign has occurred for the terminal voltage characteristics.
FIG. 3 illustrates the terminal voltage characteristics when charging a lithium type battery. Such a battery is normally charged in two steps, the first of which being carried out at constant current ("CC"). Starting at a point in time t.sub.0 the battery is hence supplied with constant current under surveillance of the terminal voltage u. When the terminal voltage reaches a certain value u.sub.1 at a point in time t.sub.1, the charging process enters the next step, namely charging at constant voltage ("CV"). The battery, which gets fully charged at a point in time t.sub.f, does not exhibit any characteristic voltage peak of the type disclosed in FIG. 2, and consequently this criterion cannot be used for controlling the charging process (particularly the ending thereof).
Since the battery types illustrated in FIG. 2 and FIG. 3, respectively, exhibit remarkably different charging characteristics, it has been difficult in the prior art to make batteries of both types available for use with the same mobile telephone model.